The bulk composition of Mars

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.     

The nature and origin of brecdation and mineralization at the White Crystal ore deposit,Ardlethan tin mine,New South Wales

The White Crystal ore zone consists of an ovoid mass of fine grained quartz‐tourmaline topaz rock containing disseminated cassiterite together with prominent vughs of quartz and/or tourmaline, cassiterite, and sulphides. Toward the margins of the ore zone the decreasing intensity of alteration illustrates formation via a process of brecciation, infill, and alteration. The rectangular slab‐like fragments are commonly aligned and their plunge indicates a dip towards a common epicentre. The tourmalinized ore zone is surrounded by successive zones of sericitization and argillization. The brecciation is of large scale tensional origin which is suggestive of collapse. Beneath the ore zone mineralization/alteration continues at subeconomic grades and limited data suggests an intrusive hydrothermal style of brecciation. The entire system is a form of breccia pipe and has yielded some 1.5 Mt of ore at around 0.64% Sn.     

A study of Holocene floodplain particle size characteristics with special reference to palaeochannel infills from the upper Severn basin,Wales, UK

Multiple sedimentary units from floodplain reaches at Welshpool on the upper River Severn and at the confluence of the Afon Tanat and Afon Vyrnwy (mid‐Wales, UK) were examined to ascertain if they have distinctive particle size characteristics. Changes in particle size characteristics and their possible relationship to known human and climatic impacts are also discussed. Ellipse plots of particle size characteristics from the River Severn floodplain at Welshpool show that coarse‐grained outwash deposits can be clearly discriminated from channel margin or palaeochannel sediments. In contrast, at the Afon Tanat–Vyrnwy study reach, this discrimination is not seen so clearly. The relationships between age and particle size characteristics from the most sampled sedimentary environment, palaeochannel infills, were also examined. The data from the River Severn floodplain at Welshpool show that palaeochannel sediments reveal a gradual but clear increase in particle size from the mid‐ to late Holocene towards the present day. Sediments deposited in the period 90–160 years BP are markedly coarser. It is suggested that these changes may be related to the combined effect of land‐use changes, metal mining impacts and changes in flood frequency and magnitude that occurred at this time within the upper Severn basin. In contrast, the particle size characteristics of post Late Devensian/Early Holocene units from Tanat–Vyrnwy palaeochannels were random with no discernible age–size patterns. It is suggested that the non‐systematic grain size distribution may be due to the steeper valley gradients of the Tanat–Vyrnwy system (and by inference higher stream powers) and its relatively narrow valley form enabling more effective coupling between coarser outwash deposits found on and at the edges of hillslopes and the valley floor. Although the two study reaches have undergone comparable environmental change during the Holocene and lie in the piedmont zone of their catchments, palaeochannel units of the same age possess distinctly different characteristics. Intrinsic reach‐scale geomorphic factors would appear to preclude the uniform application of particle size characteristics to determine alluvial response to environmental change. Consequently, care needs to be applied to the use of such data for environmental discrimination because the phenomenon of equifinality means that a specific set of sediment characteristics is not necessarily exclusive to specific fluvial environments in either space or time. Copyright © 2001 John Wiley & Sons, Ltd.     

Paricutin re-examined: a classic example of crustal assimilation in calc-alkaline magma

Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km³ of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO₂, ¹⁸O of +6.9 to 7.0, and ⁸⁷Sr/⁸⁶Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, ¹⁸O values up to +7.6, and ⁸⁷Sr/⁸⁶Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K₂O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%, ¹⁸O of +5.6 to 9.9 and ⁸⁷Sr/⁸⁶Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%, ¹⁸O of +7.7 to 8.6, and ⁸⁷Sr/⁸⁶Sr between 0.7047 and 0.7056.These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes.The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities.     

The deformation matrix and the deformation ellipsoid

Homogeneous strain can be computed most easily by the methods of matrix algebra. Lines, planes and ellipsoids represented in matrix form can be homogeneously deformed by simple matrix multiplication by linear transformation matrices, the elements of which are the coefficients of the transformation equations. Deformation matrices or linear transformation matrices which cause geological-type homogeneous strain are divided into four classes based on the presence or absence of symmetry and/or orthogonality. The nature of the homogeneous strain caused by each class of deformation matrix is examined. Orthogonal-symmetrical and orthogonal matrices cause rotation. Symmetrical matrices cause irrotational strain with co-axial strain as a special case. Matrices which are neither orthogonal nor symmetrical cause many different types of rotational strain, some of which are examined.     

The Shaw meteorite: History of a chondrite consisting of impact-melted and metamorphic lithologies

The Shaw L-group chondrite consists of three intermingled lithologies. One is light-colored and has a poikilitic texture, consisting of olivine (many skeletal and euhedral) and augite crystals surrounded by larger (up to 1 mm) orthopyroxene grains; plagioclase occurs between orthopyroxene crystals and rare, small (<5 μm) patches of Si-K-rich glass or cryptocrystalline material occurs within the plagioclase. The skeletal olivine crystals contain 0.08–0.16 wt% CaO. Petrofabric measurements show that the c-axes of the olivines are aligned. The light-colored lithology also contains numerous vugs and vesicles: SEM studies reveal euhedral, possibly vapor-deposited, crystals of olivine and pyroxene in the vugs. A second lithologic type is dark-colored, contains remnant chondrules. and has a microgranular texture. Poikilitic orthopyroxene crystals, where present, are smaller (0.1–0.2mm) than they are in the light-colored lithology. Microgranular olivine crystals contain <0.08 wt% CaO: most contain 0.03–0.05 wt% CaO. Vugs are rare and Si-K-rich material is absent. The third lithologic type is gray macroscopically and seems to be intermediate between the other two. It has a well-developed poikilitic texture, but contains neither skeletal olivines (euhedral olivines are rare) nor Si-K-rich material: remnant chondrules are present but less abundant than in the dark lithology. A modal analysis of a 5300 mm² slab shows, contrary to published opinions, that Shaw contains normal L-group chondrite abundances of metal and troilite. However, these phases are distributed irregularly throughout the meteorite. The light colored lithology is nearly devoid of metal and troilite and centimeter-sized metal-troilite globules occur between the three silicate lithologies. Wherever the metal occurs, it consists of nearly homogeneous martensite (13.9 wt% Ni) rimmed by kamacite (7.1 wt% Ni). These data indicate that Shaw is a partly-melted shock-breccia. The light-colored lithology must have been totally melted, as shown by the presence of aligned. CaO-rich, skeletal olivines; Si-K-rich residual material: and vugs and vesicles lined with euhedral crystals of mafic silicates. The dark areas appear to be unmelted target rock of L-group composition. Analysis of the growth of kamacite at the taenite (now martensite) borders indicates a cooling rate of ~ 3 C/10³ yr. or one thousand times faster than most ordinary chondntes. The Shaw impact event probably formed a crater several kilometers in diameter on its meteorite parent body.     

Megacrysts and xenoliths in kimberlite,Elliott County,Kentucky: A mantle sample from beneath the Permian Appalachian Plateau

Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo_85–93), garnet (0.21–9.07% Cr₂O₃), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56–0.88% Cr₂O₃), and Cr-poor orthopyroxene (<0.03–0.34% Cr₂O₃) in a matrix of olivine (Fo_88–92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79–0.86) and CaO (4.54–7.10%) with Cr₂O₃ content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr₂O₃ and increase in TiO₂ (0.38–0.56%) with decreasing mg (0.87–0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06–0.38% Cr₂O₃) and less Cr-rich than peridotite clinopyroxenes (1.39–1.46% Cr₂O₃). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09–1.16% CaO and 1.16–1.18% Al₂O₃) and low-Ca, low-Al (0.35–0.46% CaO and 0.67–0.74% Al₂O₃). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86–0.98% CaO and 0.95–1.01% Al₂O₃). The high-Ca, high-Al group (Group I) has lower mg (0.88–0.90) than low-Ca, low-Al group (Group II) with mg=0.92–0.93; low mg orthopyroxenes (Group Ia) have lower Cr₂O₃ and higher TiO₂ than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr₂O₃ than peridotite orthopyroxenes (0.46–0.57% Cr₂O₃). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165°–1,390° C and 1,295°–1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165°–1,255° C and 51–53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970°–1,020°C and 46–56 kb (± 5kb). Garnet peridotites equilibrated at 1,240°–1,360° C and 47–49 kb. Spinel peridotites have discordant temperatures of 720°–835° C (using spinel-olivine Fe/Mg) and 865°–1,125° C (Al in orthopyroxene).Megacrysts probably precipitated from a fractionating liquid at >150 km depth. They are not disaggregated peridotite because: (1) of large crystal size (up to 1.5 cm), (2) compositions are distinctly different from peridotite phases, and (3) they display fractionation trends. The high mg, low T orthopyroxenes and the clustering of olivine rims near Fo_89–90 reflect liquid changes to higher MgO contents due to (1) assimilation of wall-rock and/or (2) an increase in Fe³⁺/Fe²⁺ and subsequently MgO/FeO as a result of an increase in f _o.